This invention relates generally to screw presses for expressing fluids from fibrous materials, and more particularly to a bi-metallic worm assembly for use in conjunction with such presses.
The flights on worm assemblies which radially extend from the flight body of feed screws of high pressure expressing presses incur substantial wear and abusive interaction with both fibrous material and debris contained therein as they interact with the walls of the screw press. It is a well-known technique to provide wear resistant or hardfacing coatings upon the surfaces of the flight and flight body which are subjected to highest wear. Techniques utilized for this purpose are deposit welding, flame spray deposition, plasma deposition and the like. Thereafter, the surfaces are smoothed manually back to the desired dimension of the flight. These conventional deposit welding techniques are labor intensive, require expensive components, and provide poor bonding between the ductile base material and the harder deposited weld material.
Considerable effort has been expended to resolve this wear problem as described in the following U.S. and foreign patents known to applicant which include some combination of bi-metallic structure:
French--U.S. Pat. No. 3,592,128, PA0 Bredeson--U.S. Pat. No. 3,980,013, PA0 Knuth, et al.--U.S. Pat. No. 4,223,601, PA0 Theysohn--U.S. Pat. No. 4,364,664, PA0 Mansfield--U.S. Pat. No. 4,440,076, PA0 Zies--U.S. Pat. No. 3,034,424, PA0 French, et al.--U.S. Pat. No. 3,721,184,
Mansfield--U.S. Pat. No. 4,996,919, PA1 Williamson--U.S. Pat. No. 4,838,700, PA1 - - - --U.K. 592,834, PA1 - - - --Italy 557,425, PA1 Appleby--U.K. 310,680.
Several attempts have also been made to produce a homogeneous feed screw by utilizing casting techniques. However, if a highly wear resistant brittle material is chosen, cracking at the keyway or other highly stressed areas occurs. Alternately, where a more ductile material is used, premature wear of the flight is experienced.
Applicant has also invented another form of a bi-metallic feed screw as described in his co-pending application, Ser. No. 07/411,191 filed Oct. 19, 1989, now U.S. Pat. No. 4,996,919. However, this invention is directed to the mechanical engagement of a flight within a mating cavity formed the flight body itself.
The present invention utilizes the techniques of brazing and heat expansion and shrinkage to interconnect the relatively soft and tough inner hub within a worm flight cast formed of harder material such as STELLITE and a method of manufacture therefor. This structure is ideally suited for high wear resistance, minimum internal stress risers and maximized inner hub toughness and ductility.